This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Our work has provided compelling evidence that the onset of immune deficiency in pathogenic SIVmac239 infection of rhesus macaques (RM) is intimately linked with patterns of CD4+ memory T cell dynamics, and reflects a complex interplay of direct viral cytopathogenicity and the indirect effects of persistent immune activation on CD4+ memory T cell proliferation, differentiation and survival. Our data have strongly implicated a decline in effector site, CD4+ effector memory (TEM) populations below a crucial threshold as a major proximate mechanism of overt immunodeficiency, but significantly, have also demonstrated that in CCR5-tropic SIV infection, even massive viral-mediated destruction of CD4+ TEM is rarely, if ever, sufficient for functional failure of this population. Instead, our data indicate that this failure is determined in large part by the ability of CD4+ central memory T cells (TCM) to produce new TEM, which in turn depends on the maintenance of the CD4+ TCM population itself. In chronic SIV infection, we found that a slow decline of the number and often, proliferative activity of CD4+ TCM underlies CD4+ TEM population failure, and "sets the clock" for the onset of overt disease.